Higher busulfan cumulative exposure predicts better thalassemia-free survival in patients with beta thalassemia undergoing HCT with busulfan-cyclophosphamide containing conditioning regimen Free

Busulfan systemic exposure has been shown to influence HCT outcomes, and adjusting the doses to achieve target systemic exposures helps improve outcomes. Several groups have established therapeutic cut-offs for optimal busulfan exposures, and few centres are routinely performing targeted dose adjustments to achieve target cumulative exposures. We compared one-year thalassemia-free-survival in patients undergoing HCT with Bu/Cy regimen based on the cumulative busulfan systemic exposure.

A total of 89 patients with beta thalassemia who underwent HCT with a busulfan-cyclophosphamide regimen were enrolled in this study from 2012-2024. The median age of the patients was 3 years (1-9yrs), of which 58 were males. The majority of the patients belonged to Lucarelli Class II (79%), followed by Class I (13%) and Class III (most of whom had 2^nd transplant 8%). Blood samples were collected before Bu administration, at the end of infusion, and at 1hr, 2hrs and 4hrs post-infusion. Plasma was separated immediately and busulfan levels were measured using an established LC-MS assay. The AUC was calculated using 2 methods; Non-linear mixed effects modeling analysis with Monolix (version 5.1.0) using the Stochastic Approximation Expectation-Maximization (SAEM) method. We also used a model-informed precision dosing software (InsightRx), which was used to predict the subsequent doses. The subsequent doses were either escalated or reduced as and when deemed necessary based on the day's AUC. A cumulative AUC of 20000µmol*min was targeted. We then compared the total cumulative AUC with rejection, one-year overall survival, and thalassemia-free survival as outcomes. One-year thalassemia-free survival (TFS) was defined as the time from transplant to the occurrence of an event (rejection or death within a year).

An initial quartile analysis was done to identify suitable target ranges for cumulative AUC. Further analysis was based on the following four groups - <12127 (n=19), 12127 – 15473 (n=22), 15473-21488 (n=22) and >21488 (n=22)). Patient characteristics such as distribution of age, sex and Lucarelli classification were similar among the 4 groups. Graft rejection occurred only in the lower two quartiles (n=3 in <12127 and n=1 in 12127-15473). However, a Pearson chi-square analysis showed no significant difference among the four groups. Sinusoidal obstruction syndrome (SOS) occurred in a total of 14 patients, 9 of whom had Bu AUC greater than median value of 15473umol*min. However, there was only one death due to SOS in the high AUC group. A fisher’s exact test revealed no association between Bu exposure and toxicity related deaths. We then performed a Mantel-Cox Log rank test to check if a median cut-off AUC of 15473 µmol*min could predict better 1yr TFS. Achieving an AUC_cum >15473 µmol*min showed a trend towards improved survival outcome (p=0.055). Therefore, we suggest that targeting an AUC_cum >15473 µmol*min could result in better TFS and lower the possible occurrence of graft rejection.